Air pollutants such as ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2) and black carbon are monitored at thousands of locations around the world because of their adverse health effects and contributions to climate change. Of these, O3, NO2 and SO2 have been designated “Criteria Pollutants” in the U.S. with limits on their ambient concentrations regulated by the Environmental Protection Agency (EPA). Concentrations of these pollutants are regulated by many other countries as well. Black carbon is the carbonaceous or “soot” component of particulate matter (PM) and specifically PM2.5, defined as the mass per unit volume of all particles having diameters of 2.5 μm or less, which is another US EPA Criteria Pollutant. Black carbon is currently of great interest as an air pollutant because of both its adverse health effects and its contribution to climate change. For example, it has been estimated that 20% of global warming and 40% of glacier melting to date is due to black carbon, making it second only to CO2 as a driver of global climate change (Ramanathan, 2007). Because of their small size (freshly emitted particles are less than 0.1 μm), black carbon particles penetrate deep into lungs and contribute to a wide range of health problems, including asthma, and most likely cardiovascular disease and lung cancer (Janssen et al., 2012). Sulfur dioxide is a primary pollutant emitted to the atmosphere as a result of combustion of fossil fuels, especially coal. Nitrogen dioxide is formed in the atmosphere by the oxidation of nitric oxide (NO), which is produced in high temperature combustion of fuels such as internal combustion engines. NO2 also is emitted directly to the atmosphere from combustion processes, especially by diesel engines that also tend to produce high concentrations of black carbon. Since the 1950s it has been known that ozone is a secondary pollutant formed in the interaction of sunlight with volatile organic compounds (VOCs) and oxides of nitrogen (NOx=NO+NO2). In photochemical smog, ozone is formed in the NOx-sensitized oxidation of hydrocarbons where NOx serves as a photocatalyst (e.g., Birks, 1998). Ozone is not only damaging to human health, crops and natural ecosystems, it is also a significant greenhouse gas. Thus, as a result of the human health and climate impacts of these air pollutants, measurements of O3, NO2, SO2 and black carbon are needed now and will be needed far into the future. A new method based on direct long path absorbance for measurements of all of these chemical species is disclosed herein, with an emphasis on measurements of NO2, SO2 and black carbon, where significant improvements over existing methodologies are most needed.
Ozone, NO2, SO2 and black carbon all absorb at ultraviolet or visible wavelengths, and their concentrations can be measured by optical absorbance. Light absorbance is governed by the Beer-Lambert Law:
                                          [            I            ]                                [                          I              o                        ]                          =                                            e                                                -                  σ                                ⁢                                                                  ⁢                lc                                      ⁢                                                  ⁢            or            ⁢                                                  ⁢            c                    =                                    1                              σ                ⁢                                                                  ⁢                l                                      ⁢                          ln              ⁡                              (                                                      I                    o                                    I                                )                                                                        (        1        )            where Io is the light intensity passing through the detection cell with no analyte (e.g., O3 NO2, SO2, black carbon) present, I is the intensity of light passing through the detection cell when the analyte is present, σ is the extinction coefficient for the analyte (absorption cross section in cm2 molec−1 for gases; mass extinction coefficient in m2 g−1 for particulates), l is the path length through the detection cell (cm), and c is the concentration of analyte within the detection cell (molec cm−3 for gases; μg m−3 for particulates). The analyte concentration is often converted to a mixing ratio such as parts-per-million by volume (ppm) or parts-per-billion by volume (ppb) by dividing by the total concentration of air molecules and multiplying by the appropriate factor (106 for ppm and 109 for ppb). The total concentration of air molecules is usually determined by measuring the temperature and pressure within the detection cell and using the ideal gas law. Light absorbance is an especially attractive technique, since it relies only on knowing σ, which is an intrinsic property of the molecule; the path length, which is easily measured; and the ability to measure relative light intensities.
The most common method for measuring ozone is by absorbance of the 253.7 nm emission line of a low-pressure mercury lamp. The absorbance, ln(Io/I), can be measured in modern photometers with a precision (standard deviation of the noise or RMS noise) of typically ˜3×10−6 for 10-second averaging times. Combining this with the ozone absorption cross section and optical path length determines the overall precision expected for a measurement of a given analyte at ambient temperature and pressure:
                              Precision          ⁡                      (            ppb            )                          =                                            3              ×                              10                                  -                  6                                                                    σ              ⁢                                                          ⁢                              l                ⁡                                  (                                      P                    ⁢                                          /                                        ⁢                    kT                                    )                                                              ×                      10            9                                              (        2        )            Here, σ is the absorption cross section, P is the total pressure, k is the Boltzmann constant, T is the absolute temperature, and the factor of 109 converts the mole fraction to parts-per-billion by volume (ppb). The absorption cross section for ozone is 1.15×10−17 cm2/molec, and the value of P/kT (the total concentration of gas molecules) is 2.46×1019 molec/cm3 at 1 atm of pressure and temperature of 25° C. For ozone, the precision is calculated to be 0.7 ppb for a path length of 15 cm and 0.35 ppb for a path length of 30 cm, in good agreement with the performance of commercial ozone monitors currently on the market.
The air pollutants NO2, SO2 and black carbon absorb much less strongly than ozone, with absorption cross sections being ˜6×10−19 cm2/molec for NO2 (Burrows et al., 1998) at 405 nm and ˜7×10−19 cm2/molec for SO2 at 290 nm (Vandaele et al., 1994). Rearranging equation 2, it may be calculated that to obtain a precision of 1 ppb for NO2 would require a path length of ˜203 cm and for SO2 would require a path length of ˜174 cm.
Mass extinction coefficients for particulate matter are commonly expressed in units of m2/g. The mass extinction coefficient for black carbon, a form of particulate matter, depends on size distribution and other physical properties, and is typically cited to be in the range 5-20 m2/g at 880 nm with a recommended value of 7.7 m2/g (Bond and Bergstrom, 2006). Assuming this extinction coefficient and again assuming the precision in the measurement of absorbance to be 3×10−6 for 10-s averaging times, a path length of 3.9 m (390 cm) would be required to obtain a precision of 0.1 μg/m3 for black carbon mass concentration, calculated as 3×10−6/(7.7 m2/g×0.1 μg/m3×10−6 g/μg). Data averaging can be used to further improve the precision, and that improvement can be traded for a shorter path length. For example, averaging for 1 minute (six 10-s measurements) typically will improve the precision by a factor of √5 or 2.24, allowing the path length to be reduced to 174 cm for the measurement of black carbon with a precision of better than 0.1 μg/m3.
Because of the long path lengths required, calculated above to be 1.7-3.9 meters to obtain necessary measurement precisions for 10-s averaging, the pollutants NO2, SO2 and black carbon are seldom measured by direct absorbance in the gas phase. The most common method to measure NO2 has long been reduction to NO, usually by passing through a heated molybdenum catalyst bed (Winer et al., 1974), followed by detection of its chemiluminescence with ozone at reduced pressure (Fontijn et al., 1970). A more recent method makes use of photolytic conversion of NO2 to NO with blue light near 405 nm (Buhr, 2007). A major disadvantage of the chemiluminescence method is that the conversion efficiency is not typically 100% and varies with time. In the case of the molybdenum converter, other nitrogen species in the atmosphere, especially peroxyacetylnitrates (PANs), N2O5 and nitric acid (HNO3), may be converted as well (Winer et al., 1974). The photolytic conversion method is less efficient (typically ˜50%) due to a photochemical equilibrium established within the photoreactor between NO, NO2 and O3, and, as a result, the conversion efficiency depends on the ambient concentration of ozone. Furthermore, regardless of the conversion method, the measurement of NO2 is indirect, being calculated from the difference between measurements of NOx (NO2+NO) obtained by passing through the converter and measurements of NO without passing through the converter.
Sulfur dioxide has typically been measured by fluorescence (Schwarz et al., 1974). However, absorbance has the advantage of being an absolute method, requiring no or only infrequent calibration. As noted earlier, for absorbance measurements, only relative light intensities need to be measured since the Beer-Lambert Law requires only the ratio of light intensities, III, in order to calculate the analyte concentration from the absorption cross section and easily known path length. Instruments based on absorbance are typically less expensive to construct than fluorescence-based instruments and require less power because a high intensity light source is not required. Thus, an instrument based on direct absorbance of SO2 would have advantages over fluorescence, at least in those applications where it provides adequate sensitivity. One example is smokestack monitoring for SO2 emissions in the combustion of fossil fuels such as coal or natural gas, where a more robust instrument requiring little maintenance and infrequent calibration is desirable.
Black carbon has long been measured by the method of aethalometry developed in the early 1980s, whereby particulate matter is continuously deposited on a filter and transmission of light through the filter is continuously measured (Hansen et al., 1982). However, aethalometers have been demonstrated over the past few years to have several artefacts associated with the requirement that particles be pre-concentrated by continuous collection on a filter tape (Weingartner et al., 2003; Arnott et al., 2005; Baumgardner et al., 2012). Light scattering within the filter matrix increases the extinction by a variable factor of ˜2, and co-deposited particles of other types increase light scattering as well. Also, the agglomeration of particles within the filter changes their fundamental optical properties. Thus, it is highly desirable to measure black carbon by direct absorbance in the gas phase without pre-concentration on a filter medium.
Several approaches to the development of instruments for long path absorption measurements of species in the gas phase have been taken in the past. By use of mirrors, the path can be folded within a detection cell with up to 100 or more reflections, thus greatly increasing the absorption path length. Of these, the White cell has been one of the most successful and can be applied to both collimated and uncollimated light beams (White, 1942). A disadvantage of this approach, however, is that even the miniaturized versions of White cells have relatively large volumes, typically 180 cm3 and larger, so that the flush times for typical flow rates of 1.8 L/min (30 cm3/s) are long. Also, the cell shapes required by the mirror arrangements necessitate multiple flush times. Assuming exponential dilution, a detection cell requires ˜4.6 flush times to exchange 99% of its contents. Thus, for a cell volume of 180 cm3 (volume of a currently commercially-available miniature White cell with 2-meter path length) and flow rate of 30 cm3/s, the total required flush time is 4.6×180/30=27.6 s. In order to obtain the low absorbance precisions of 3×10−6 stated earlier, it is important to measure the reference light intensity (Io) every 5 to 10 seconds due to small intensity drifts in typical light sources. This requires total cell flushing times of 2.5 to 5 seconds (to measure both I and Io), which is incompatible with the miniature White cell described above unless excessively large flow rates (>10 L/min) are used. However, large flow rates are impractical because of the size, weight and power consumption of the air pump required, and the large scrubber capacity necessary to quantitatively remove the analyte from such a large flow rate for the reference measurement. For this reason, typical flow rates used in commercially-available air pollution monitors are in the range 0.5-3 L/min.
Herriott cells (Herriott and Schulte, 1965) may be used to achieve very long path lengths of 50 m or more for absorbance measurements, and although low volume Herriott cells have been demonstrated, they require a laser as a light source to achieve the required degree of collimation. Unfortunately, the advantage in sensitivity gained by the longer path lengths achievable using Herriott cells are largely offset by the greater noise of a laser light source as compared to uncollimated sources, such as light emitting diodes or low-pressure mercury lamps that can be used with White cells.
Accordingly, 2B Technologies, Inc. developed another approach, a so-called “Folded Tubular Photometer” (e.g., 2B Technologies, Inc.'s Model 405 nm NO2/NO/NOx Monitor), for measurements of a pollutant or other species in a gas such as air. This type of device uses modular mirror cubes in combination with modular tubular detection cells, which allow the light path to be folded and make it compact enough for a several-meters-long detection cell to fit into a conventional rack-mount-sized or smaller enclosure (rack-mount is a common term in the art meaning the equipment will fit in a conventional electronic rolling rack having 19-in wide slots). Further, this approach makes it possible to reduce the cell volume and therefore also the flush times significantly, compared with the White cell described above. Typically, tubular detection cells were used with a 3/16 in (0.476 cm) inner diameter (i.d.) such that a 2-m long path length has a calculated volume of only ˜35.6 cm3. Thus, the time for one flush at a flow rate of 1.8 L/min (30 cm3/s) is only 35.6/30=1.2 s. The time for a molecule to diffuse across the inner diameter of the tubular detection cell is calculated to be ˜0.5 s, and therefore, nearly plug flow results and only one or two flush times are required to achieve greater than 99% complete flushing of the previous contents of the detection volume. This allows a new I or Io measurement to be made once every 5 s or less, and because those measurements are made close together in time, variations in the lamp intensity between measurements is small. As a result, higher precision can be achieved than in a White or Herriott cell of the same path length. With Folded Tubular Photometers, measurements of ambient concentrations of NO2, SO2 and black carbon by direct absorbance in the gas phase thus become feasible.
In addition to increasing precision by allowing measurements of I and Io close in time, the low-volume Folded Tubular Photometer allows rapid measurements of both NO2 and NO within the same instrument and temporally separated by only a few seconds. NO is measured by addition of ozone to convert NO to NO2 with near 100% conversion by the reaction:NO+O3→NO2+O2  (3)Subsequent measurement of the increase in NO2 concentration upon addition of ozone provides a highly accurate measurement of NO. Alternative commercially-available methods based on absorbance such as cavity attenuated phase shift spectroscopy (CAPS) (Kebabian et al., 2005) measure NO2 but not NO, most likely because a large cavity (detection cell) is required that cannot be rapidly flushed with the practical flow rates of 1-3 L/min employed.
However, in attempting to use such low-volume, long-path tubular detection cells, including with the light beam folded using mirrors or unfolded, substantial errors were consistently encountered in the concentration of analytes determined from the I and Io measurements compared to known values of the sample gas. For example, an error of ˜50 ppb was typically found in the measurement of NO2 at a flow rate of ˜1.8 L/min through a Folded Tubular Photometer having a path length of ˜2 m and cell volume ˜35.6 cm3. Therefore, despite the expected benefits of the Folded Tubular Photometer over the state of the art discussed above, the measurement errors obtained rendered this approach unacceptable for applications requiring accurate measurements of pollutants in a sample gas.
The foregoing examples of the related art and limitations therewith are intended to be illustrative and not necessarily exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.